APOD: Perihelion to Aphelion (2020 Jan 09)

[APOD Robot]

Perihelion to Aphelion

Explanation: Perihelion for 2020, the point in Earth's elliptical orbit when it is closest to the Sun, occurred on January 5th. The distance from the Sun doesn't determine the seasons, though. Those are governed by the tilt of Earth's axis of rotation, so January is still winter in the north and summer in southern hemisphere. But it does mean that on January 5 the Sun was at its largest apparent size. This composite neatly compares two pictures of the Sun, both taken from planet Earth with the same telescope and camera. The left half was captured on the date of the 2020 perihelion. The right was recorded only a week before the July 4 date of the 2019 aphelion, the farthest point in Earth's orbit. Otherwise difficult to notice, the change in the Sun's apparent diameter between perihelion and aphelion amounts to a little over 3 percent. The 2020 perihelion and the preceding 2019 aphelion correspond to the closest and farthest perihelion and aphelion of the 21st century.

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[TheZuke!]

This has me wondering, although not really on topic with today's APOD.

I don't really understand how the latest sunrise, and earliest sunset are not on the day of the Winter Solstice (or so I've heard),
and conversely; the latest sunset, earliest sunrise and the Summer Solstice. (local time, Northern Hemisphere, in my case.)
Somewhere along the line, I think I read it had to do with the Earth's precession, or with the 23 degree of the Earth's axis, but I don't "see" how it all fits together.

Nor do I "get" why the annalema (sp?) has a larger loop and is not symmetrical.

Thanks to all who will enlighten (no pun intended) me on these subjects.

[Keyman]

the change in the Sun's apparent diameter between perihelion and aphelion amounts to a little over 3 percent.

So..."super sun"?

[pferkul]

Neat comparison!

The diameter of the sun as measured from the image only increases by 1.4% from aphelion to perihelion (attached). The measured area increases by 2.8%, which is also the measured increase in "apparent diameter."

It is now apparent to me that "apparent diameter" is not the diameter as it appears in the image. The casual reader might conclude from the figure caption that the diameter of the sun looks 3% larger at perihelion, which is not true. Probably better to use "angular diameter" to reduce confusion.

[MarkBour]

Sometimes I just have to verify the basics of what's being said ... sorry.
If I understood the caption (and the data table in the link), then a few days ago, Earth was the closest to the Sun that it will get for at least a century in either direction. And it was the farthest from the Sun that it will get for at least a century in either direction just last July 4.

So, this year, the Earth is being very eccentric in its behavior. It needs to cut back on the drinking, perhaps.

[Chris Peterson]

Sometimes I just have to verify the basics of what's being said ... sorry.
If I understood the caption (and the data table in the link), then a few days ago, Earth was the closest to the Sun that it will get for at least a century in either direction. And it was the farthest from the Sun that it will get for at least a century in either direction just last July 4.

So, this year, the Earth is being very eccentric in its behavior. It needs to cut back on the drinking, perhaps.

Not quite. It's not a century in either direction, it's the 21st century. That is, from 2001-2100. And you'll note it's not all that eccentric in behavior, as the variations in perihelion and aphelion distance are extremely small in comparison with the mean distances.

[neufer]

Analemma as seen at noon GMT from the Royal Observatory, Greenwich

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This has me wondering, although not really on topic with today's APOD.

I don't really understand how the latest sunrise, and earliest sunset are not on the day of the Winter Solstice (or so I've heard),
and conversely; the latest sunset, earliest sunrise and the Summer Solstice. (local time, Northern Hemisphere, in my case.)

Somewhere along the line, I think I read it had to do with the Earth's precession, or with the 23 degree of the Earth's axis, but I don't "see" how it all fits together.

Nor do I "get" why the annalema (sp?) has a larger loop and is not symmetrical.

Analemma (from Greek ἀνάλημμα analēmma "support")

...............................................................................
OK...let's concentrate on a simply analogy:

• 1) The Earth rotates (west to east) on its axis (at a constant rate) once every 23 hours 56 minutes.
  2) The Sun "revolves" (west to east) around the Earth (at a constant rate of)
  once every 360 (24 hour) days at an angle of 23.5º.

Whenever the Sun crosses the equator at an skewed angle of 23º
it must travel 1.090º = 1.0º x sec(23.5º) to reach the next Longitude line.
But, whenever the Sun reach a Latitude of 23.5º
it need only travel 0.917º = 1.0º x cos(23.5º) to reach the next Longitude line.

Ergo:

• 1) twice a year (around the equinoxes) the Sun is moving slowly across Longitude lines
  2) twice a year (around the solstices) the Sun is moving rapidly across Longitude lines

The Earth is allowed an extra 4 minutes each day to catch up with the Sun to complete a Solar day.

• That extra 4 minutes allows the Earth to pass the slow equinox Sun
  such that the noontime Sun slips farther & farther to the West.
  However, that extra 4 minutes doesn't allows the Earth to catch the fast solstice Sun
  such that the noontime Sun slips farther & farther to the East.

This explains the semi-annual figure 8 analemma pattern.
...............................................................................
The asymmetry comes from the Earth's elliptical orbit which makes:

• 1) the winter solstice Sun "revolve" extra fast
  2) and the summer solstice Sun "revolve" not quite so fast.

[TheZuke!]

Thanks neufer!
The maths are difficult for me, but the pictures help explain it.

So... the "larger loop" is because of axis angle, the asymmetry of sunrise/sunset* is due to the elliptical orbit.
(I didn't know the asymmetry of of the sunset was not equally "distant" from the solstice either!)

*I guess that is a video cue for you! B^)

[orin stepanek]

From what I gather is that the orbit of the Earth is pretty circular!

[Chris Peterson]

From what I gather is that the orbit of the Earth is pretty circular! :shock:

Earth's orbit compared with a circle

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[TheOtherBruce]

Not quite. It's not a century in either direction, it's the 21st century. That is, from 2001-2100. And you'll note it's not all that eccentric in behavior, as the variations in perihelion and aphelion distance are extremely small in comparison with the mean distances.

Would these variations be due to things like perturbations caused by Jupiter and Saturn? I know the planets' orbits aren't quite completely stable in the long term (millions of years or more), but I didn't realise the changes would be noticeable over really short periods like a century.

[Chris Peterson]

Not quite. It's not a century in either direction, it's the 21st century. That is, from 2001-2100. And you'll note it's not all that eccentric in behavior, as the variations in perihelion and aphelion distance are extremely small in comparison with the mean distances.

Would these variations be due to things like perturbations caused by Jupiter and Saturn? I know the planets' orbits aren't quite completely stable in the long term (millions of years or more), but I didn't realise the changes would be noticeable over really short periods like a century.

Yes. Mostly Jupiter. No naturally occurring system with more than two bodies will produce "perfect" Keplerian orbits.

[TheOtherBruce]

Yes. Mostly Jupiter. No naturally occurring system with more than two bodies will produce "perfect" Keplerian orbits.

Until someone manages to solve the general 3-body and n-body problems.

[Chris Peterson]

Yes. Mostly Jupiter. No naturally occurring system with more than two bodies will produce "perfect" Keplerian orbits.

Until someone manages to solve the general 3-body and n-body problems.

No solution is possible outside of numerical simulation. Not that it matters. A solution would only confirm the lack of perfection, not eliminate it.

[MarkBour]

Sometimes I just have to verify the basics of what's being said ... sorry.
If I understood the caption (and the data table in the link), then a few days ago, Earth was the closest to the Sun that it will get for at least a century in either direction. And it was the farthest from the Sun that it will get for at least a century in either direction just last July 4.

So, this year, the Earth is being very eccentric in its behavior. It needs to cut back on the drinking, perhaps.

Not quite. It's not a century in either direction, it's the 21st century. That is, from 2001-2100. And you'll note it's not all that eccentric in behavior, as the variations in perihelion and aphelion distance are extremely small in comparison with the mean distances.

Ah, thanks.

[MarkBour]

Not quite. It's not a century in either direction, it's the 21st century. That is, from 2001-2100. And you'll note it's not all that eccentric in behavior, as the variations in perihelion and aphelion distance are extremely small in comparison with the mean distances.

Would these variations be due to things like perturbations caused by Jupiter and Saturn? I know the planets' orbits aren't quite completely stable in the long term (millions of years or more), but I didn't realise the changes would be noticeable over really short periods like a century.

Yes. Mostly Jupiter. No naturally occurring system with more than two bodies will produce "perfect" Keplerian orbits.

It may be that Jupiter is the most influential planet for this, but I note that in the text accompanying the table (see the last link in the APOD caption) one finds the statement:

Due of the gravitational perturbation of the Moon (and to a much lesser extent the planets), Earth's actual distance at perihelion can vary from 0.9831914 AU (147,083,346 km) to 0.9833860 AU (147,112,452 km).

That leads me to believe that the timing of the lunar orbit versus Earth's orbit is the most influential factor causing the variations.

[MarkBour]

This has me wondering, although not really on topic with today's APOD.

I don't really understand how the latest sunrise, and earliest sunset are not on the day of the Winter Solstice (or so I've heard),
and conversely; the latest sunset, earliest sunrise and the Summer Solstice. (local time, Northern Hemisphere, in my case.)
Somewhere along the line, I think I read it had to do with the Earth's precession, or with the 23 degree of the Earth's axis, but I don't "see" how it all fits together.

Nor do I "get" why the annalema (sp?) has a larger loop and is not symmetrical.

Thanks to all who will enlighten (no pun intended) me on these subjects.

I don't have anything to contribute here ... I like neufer's answer, that essentially says that the timing we picked, of consistent 24-hour days, versus the rate or the Sun's apparent traversals throughout the year, is the cause of the behaviors you asked about. And I certainly once began with the same expectations you had, only to find out with observation, study and surprise, why the guesses were wrong.

But your comments made me think of an annoying trick question:
On what date is the earliest and the latest sunrise each year?


"January 1 and December 31, of course." would be the smart-aleck answer.

[Chris Peterson]

Would these variations be due to things like perturbations caused by Jupiter and Saturn? I know the planets' orbits aren't quite completely stable in the long term (millions of years or more), but I didn't realise the changes would be noticeable over really short periods like a century.

Yes. Mostly Jupiter. No naturally occurring system with more than two bodies will produce "perfect" Keplerian orbits.

It may be that Jupiter is the most influential planet for this, but I note that in the text accompanying the table (see the last link in the APOD caption) one finds the statement:

Due of the gravitational perturbation of the Moon (and to a much lesser extent the planets), Earth's actual distance at perihelion can vary from 0.9831914 AU (147,083,346 km) to 0.9833860 AU (147,112,452 km).

That leads me to believe that the timing of the lunar orbit versus Earth's orbit is the most influential factor causing the variations.

Yes, that could be right. It makes some sense if you look at the slightly scalloped shape of Earth's orbit because of the Moon. It would be interesting to look at the path of the Earth-Moon barycenter rather than just the path of the Earth.

On the other hand, the variation in aphelion (or perihelion) distance is well over 10,000 km, which is quite a bit more than the distance between Earth's center and the barycenter of the Moon. And the Earth rotates around a point that is near the surface of the Sun, not the Sun's center. And Jupiter causes the position of the Sun to wobble with respect to the barycenter of the Solar System. Throw all of that together, and without a lot of analysis, I'm reluctant to say just what has the most influence.

[twymer]

maybe it is just me but the photo shows an increase only on the northern pole and not an equal amount on the southern pole - shouldn't the apparent increase be equal on both poles?

[Chris Peterson]

maybe it is just me but the photo shows an increase only on the northern pole and not an equal amount on the southern pole - shouldn't the apparent increase be equal on both poles?

The two images could be aligned from the bottom or from the center. Given the small difference in size, I expect the author chose the latter because it makes that difference more apparent. Aligned from the center, it might not even be seen, especially on a small screen.

[neufer]

maybe it is just me but the photo shows an increase only on the northern pole and not an equal amount on the southern pole - shouldn't the apparent increase be equal on both poles?

The two images could be aligned from the bottom or from the center. Given the small difference in size, I expect the author chose the latter because it makes that difference more apparent. Aligned from the center, it might not even be seen, especially on a small screen.

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14 girls

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...or 15 girls?

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Click to play embedded YouTube video.

[TheZuke!]

Okay,
After thinking about my previous response to neufer's response to my analemma/latest sunrise/earliest sunset question, and Chris Peterson's
diagram of the Earth's elliptical orbit, I was wrong to think the elliptical orbit was responsible for the latest sunrise/earliest sunset question.
The 23.5 degree tilt of the Earth's axis is (also) the answer to that question.
Now, I'm thinking (dangerous, I know) the asymmetry of the analemma depends on the point of view (i.e. one's latitude of the observation).
So, please correct me if I'm wrong, the analemma would be symmetrical as observed from the Equator? And larger loop (of the analemma) would be opposite in the Southern Hemisphere?

[neufer]

---

After thinking about my previous response to neufer's response to my analemma/latest sunrise/earliest sunset question, and Chris Peterson's diagram of the Earth's elliptical orbit, I was wrong to think the elliptical orbit was responsible for the latest sunrise/earliest sunset question. The 23.5 degree tilt of the Earth's axis is (also) the answer to that question. Now, I'm thinking (dangerous, I know) the asymmetry of the analemma depends on the point of view (i.e. one's latitude of the observation). So, please correct me if I'm wrong, the analemma would be symmetrical as observed from the Equator? And larger loop (of the analemma) would be opposite in the Southern Hemisphere?

The asymmetric analemma printed on Globes is the same analemma for the entire Globe (especially for folks on the equator). The asymmetry depends on the Earth's elliptic orbit and the fact that perihelion closely corresponds to Northern Winter Solstice.

Folks down-under even observe the exact same asymmetric analemma in the sky only upside down since they are all standing on their head: https://apod.nasa.gov/apod/ap150923.html

[TheZuke!]

Thanks again, neufer!
How are things in Tralfamadore?
(Not, how were things in Tralfamadore, or how will things be in Tralfamadore.)
B^)

[neufer]

Neufer designed IR filters for NOAA's GOES Sounder

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How are things in Tralfamadore?
(Not, how were things in Tralfamadore, or how will things be in Tralfamadore.)

So it GOES